The present invention relates to catalysts of the type wherein a catalytically promoting component is distended upon a monolithic support member having a plurality of fluid flow channels extending through it. The catalytically promoting component is disposed on the walls of the fluid flow channels so that a fluid such as a gas flowing therethrough comes into contact with the catalytically promoting material. The present invention is particularly adapted to provide a support suitable for the treatment of automotive exhaust gases, although it will be appreciated that it is not necessarily limited thereto but is generally suitable for catalytic contacting of fluids, such as, for example, catalytic treatment of gases including pollution abatement, catalytic processing, and catalytic combustion of fuels.
Monolithic support members having a plurality of fluid flow passages therethrough are, of course, well known in the art. For example, see U.S. Pat. Nos. 3,441,381 and 3,565,830, both assigned to the assignee of this application, the disclosures of which include catalyst carrier members of the type generally referred to as monolithic or honeycomb members. These carriers comprise inert, solid unitary or monolithic skeletal bodies having a plurality of unobstructed fluid flow channels formed therein and extending therethrough in the intended direction of fluid flow through the carrier. The carriers are preferably formed of a substantially chemically inert, generally catalytically inactive, rigid solid material. The material is sufficiently refractory to maintain its shape and sufficient mechanical strength at temperatures on the order of up to 1100.degree. C. or more so as to enable use of the carriers in the treatment of automotive exhaust gases or other high temperature applications. The fluid flow channels are disposed generally parallel to each other and extend through the carrier from one to the other opposite end faces thereof, the fluid flow channels being defined and separated from each other by a plurality of channel walls.
Generally, in order to minimize pressure drop sustained by fluids passed through the flow channels, it is desired to have a maximum amount of open fluid flow area in the end faces. To this end, the walls of the fluid flow channels are usually formed as thin as is feasible consistent with a degree of mechanical strength and integrity under thermal stress sufficient for the contemplated use. Refractory materials generally suitable to form such a carrier are materials such as zircon-mullite, alpha alumina, sillimanite, magnesium silicates, zircon, petalite, spodumene, cordierite, alumino-silicates, mullite and the like. As indicated in the above-mentioned U.S. Pat. No. 3,565,830, the disclosure of which is incorporated by reference herein, for certain applications it is preferred or essential that the carrier be essentially crystalline in form and have considerable accessible porosity.
Generally, a preferred refractory carrier is a solid, unitary or monolithic skeletal body constructed of a substantially chemically inert, substantially catalytically-inactive, rigid, solid material which is unglazed and has considerable accessible porosity. The channel walls of the fluid-flow passages preferably contain macropores in communication with the flow channels to provide increased accessible catalyst surface when the carrier is coated with a catalytic material. The geometric surface area, including the surfaces of the fluid flow channels, of a typical monolithic carrier (assuming a smooth, nonporous surface) may be on the order of 0.001 to 0.01 square meters per gram. However, the actual surface area of the carrier, taking into account the porosity of the carrier material is usually many times greater, eg., 50 to 150 or more square meters per gram, so that much of the catalytic reaction will take place within the large pores. Preferably, the skeletal structure has a macropore distribution such that over 95% of the pore volume is provided by pores of a diameter of over 2,000 Angstroms, and over 5% of the pore volume is provided by pores having a diameter of over 20,000 Angstroms. For example, in one preferred embodiment, over 20% of the pore volume is provided by pores having a diameter of over 20,000 Angstroms. The total surface area, including the pores of the carrier, is preferably about 0.08 to 6 square meters per gram, preferably about 0.2 to 2 square meters per gram.
It is known in the prior art, as shown by the abovementioned U.S. Pat. No. 3,565,830 (e.g., column 7, line 72 to column 8, line 2 thereof) that the cross-sectional shape of the fluid flow channels can be in the shape of triangles, rectangles, squares, sinusoids, circles or other circular shapes, so that the cross sections of the support represent a repeating pattern of a lattice or honeycomb type structure. At column 8, line 2 to line 8 of the same patent, it is stated that cross sections with sharp, acute angled corners are not preferred as they can collect solids such as lead compounds from the gases and become plugged and/or catalytically inactive. It is further stated that the walls of the cellular channels are generally made of the minimum thickness necessary to provide a strong unitary body. Typical wall thicknesses are exemplified as ranging from about 2 to 25 mils.
U.S. Pat. No. 4,102,980 discloses a catalytic contact apparatus for the removal of harmful components from waste gases from stationary sources such as combustion furnaces, and shows in FIG. 8 thereof a cross section of one embodiment having substantially square cross section shaped gas flow channels. The drawing somewhat schematically indicates a slight rounded shape at the corners of the channels. The patent is silent with respect to the slightly rounded corners, which feature is believed to be a typical irregularity of manufacturing processes used to make such carriers as described in more detail below with respect to one of the prior art embodiments illustrated.
One difficulty with prior art structures which employ a round or oval cross section configuration of the fluid flow channels is that portions at least of the walls defining and separating the channels are necessarily thicker than the minimum required thickness of the wall which occurs at the closest spacing of the periphery of adjacent channels. Thus, the end face wall area is undesirably increased at the expense of the open fluid flow end face area provided by the gas flow channel openings. This problem can be avoided by employing flow channels of polygonal cross section, e.g., rectangular (including square), triangular or hexagonal cross section shaped flow channels. See, for example, FIG. 4 of U.S. Pat. No. 3,910,770. Cross sectional shapes such as rectangular or hexagonal can be arranged with a generally uniform minimum wall thickness between adjacent channels thereby avoiding an undesirable increase in end face wall area as opposed to end face open flow area. However, such flow channel polygonal cross section shapes have the disadvantage that they necessarily provide sharp angular corners which define in cross section either obtuse angles (octagonal cross section) right angles (rectangular cross sections) or acute angles (triangular cross sections). The angular corners present a problem when catalytic promoting materials and/or coatings to support the same are applied to the catalytic carriers. Significant amounts of the coating and catalytically promoting materials are found to accumulate in the sharp angular corners to a depth which tends to effectively prevent access of the treated fluid to the most deeply "buried" portions of the coating and catalyst in the corners. This renders a small but significant percentage of the coating and/or catalytic material inaccessible to the fluid to be treated, thereby resulting in a general inefficiency and waste. The problem is particularly acute in an economic sense when the catalytic promoting material is a precious metal because of the high cost of the wasted catalytic material. Further, the initial accumulations of the coating and/or catalytic material in the corners tends to cause the coating on the flat planar surfaces between the corners to be correspondingly thinner, therefore exposing a lesser amount of catalytic material to the fluid flow. Even if it were economically or technically feasible to continue to build up the thickness of the coating (it is not), the coating would still be deeper in the corners than on the flat surfaces intermediate adjacent corners. It is an object of the present invention to overcome the problems such as those described above.